Noise is generated with the exhaust of an internal combustion engine for vehicles. Such exhaust noise is caused: by pressure waves generated by the difference between the pressure in the cylinder and that in the exhaust pipe at the moment when the exhaust valve is opened; by resonance in the pipe amplified and generated when the pressure waves reciprocate within the pipe; and by air current, i.e., the current of the exhaust.
An expansion type muffler, resonance type muffler, interference type muffler, sound absorption type muffler, and muffler of a type combining the above mentioned types have already been proposed as a means for eliminating exhaust noise.
In the expansion type muffler, a muffler provided with an expansion chamber of a large volume is connected to piping forming an exhaust system so that the exhaust may be introduced into the expansion chamber, to be expanded, interfered with, noise absorbed, and rectified in the pulsation in this chamber to obtain a silencing effect.
In such expansion type muffler, in the case of introducing the exhaust into the expansion chamber, the exhaust pulsates with the intermittent opening and closing of the exhaust valve of the engine, and the pressure in the expansion chamber of the muffler fluctuates. It has been known that the chamber wall is vibrated by such pressure fluctuation, and that the pulsation pressure within the chamber and the natural frequency of the wall produces sound waves by this vibration so as to be a part of the noise of the exhaust system. To prevent this, the rigidity of the outer wall of the muffler must be improved to provent the vibration, and also the natural frequency of the outer wall must be dispersed as much as possible.
The conventional expansion type mufflers are explainged hereinbelow.
In FIG. 7, a flat partition plate 4 is provided in the intermediate portion in the lengthwise direction of a cylindrical muffler body 1. A pipe 5 for introducing the exhaust is provided in a front wall 2 of the body 1, and a pipe 6 for discharging the exhaust is provided in a rear wall 3. With the partition plate 4, the body 1 is sectioned in the front and rear in the lengthwise direction to have two expansion chambers 8 and 9 which communicate with each other through a connecting pipe 7 provided through the partition plate 4. The exhaust is introduced into the first chamber 8 through the pipe 5 and expands within chamber 8. At the same tine, the sound wave component also expands within the chamber 8, collides with the inner wall and partition plate, and is reflected to be interfered with and attenuated. Then, the exhaust is further introduced into the second chamber 9 through the connecting pipe 7. The above is repeated, and the sound waves are interfered with and attenuated in turn to obtain a silencing effect.
However, the exhaust introduced into the chamber 8 contains a pulsation pressure corresponding to the intermittent opening and closing of the exhaust valve of the engine disposed in the up-stream exhaust side, and therefore the pressure within the chambers 8 and 9 pulsates therewith. Due to this pressure fluctuation, the wall of the body 1 forming the inner wall of the chamber 8 is flexed and displaced with a predetermined amplitude inwardly as shown in the drawing wherein the high rigidity parts a and b are as nodes and the low rigidity part c is as a web or anti-node.
As a result, the vibrations of the two nodes become so large as to cause noises or so-called vibration sounds of the outer wall. In addition, the partition plate 4 is provided in the central part within body 1 so that the lengths of the wall of the body 1 from the front and rear walls 2 and 3 to the partition plate are equal to each other over the entire periphery of the body. Therefore, the wall of body 1 results in having equal natural frequencies as well as the same vibration nodes over the entire periphery thereby making the vibration sounds much greater. Further, because the partition plate 4 is flat, the plate vibrates itself the same as is mentioned above within the body 1, with its middle part acting as a web, and causes internal noises separately from the vibrations of the wall. Noise in the muffler is thus amplified together with the above.
In FIG. 8, a partition plate 104 is provided to traverse in the lengthwise direction in a body 101. The body 101 is sectioned above and below with the partition plate 104 to provide upper and lower expansion chambers 108 and 109. The front and rear walls 102 and 103 are provided respectively with pipes 105 and 106. The chambers 108 and 109 are connected respectively to the upper stream side and lower stream side through the respective pipes 105 and 106. The partition plate 104 is provided with a pipe 107 which makes the chambers 108 and 109 communicate with each other.
The outer wall of the body 101 vibrates as flexed and displaced inwardly and outwardly with the front and rear ends in the lengthwise direction being as knots a and the intermediate part being as a web c, and further the partition plate 104 disposed in substantially parallel with an outer wall vibrates to bring about the same problems as mentioned above.
The present invention eliminates the aforementioned problmes of the expansion type mufflers by taking into consideration that the noise can be prevented as much as possible by increasing the rigidity of the partition plate, reinforcing plate, and outer wall of the muffler to control the vibration and disperse the natural frequency of the outer wall.